This invention relates to a caging arrangement for a center port valve located in a bore of a master cylinder to control communication between the bore and a reservoir.
U.S. Pat. Nos. 5,018,353 and 5,111,661 disclose master cylinders wherein compensation between a first chamber in a bore of a housing and a reservoir occurs through a single center port compensation valve associate with a first piston and between a second chamber in the bore through a relationship established between a radial port and sealing structure carried on a second piston. In order to eliminate the possibility of cutting a nub in a seal by extrusion into the radial port, U.S. Pat. Nos. 5,207,062; 5,279,125 and 5,943,863 disclose the use of a second center port compensation valve through which compensation is achieved for the second operational chamber in a bore. Most such master cylinders have a separate compensation valve for the first and second pressurizing chambers in the bore through which communication typically occurs through ports when the first and second pistons are in a rest position. As the first and second pistons approach the rest position, a stop pin engages the compensation valve to open the compensation valve and initiate communication between the bore and reservoir. Unfortunately, the passage required to connect the rear chamber is quite long as the compensation port needs to be located adjacent the end of the housing of the master cylinder. In addition, when such master cylinders are recessed into a front chamber of a vacuum brake booster care needs to be taken with respect to sealing structure to assure that vacuum does not draw fluid into the vacuum brake booster. While this type master cylinder functions in an adequate manner under some circumstances, the fluid pressure generated during a brake application may extrude a portion of a poppet into a compensation port leading to a reservoir and as a result it may be possible to cut the face on the poppet to an extent that a leak may develop between the bore and the reservoir during a brake application. The invention in U.S. patent application Ser. No. 10/246,101 filed Sep. 18, 2002, discloses structure for reducing the possibility of damage to a face seal of a poppet, however, in many master cylinders that have center port compensation valves the process of bleeding air from a brake system can present a challenge when caging pins for the return spring move within a blind passage of a piston and as a result care must be taken to assure that the system is completely filled with fluid.
A primary object of the present invention is to provide a brake system having a master cylinder with a center port compensation valve wherein a head on a stem for caging a return spring moves within a poppet assembly as a direct function of the compression of the return spring during the development of pressurized fluid that is supplied to wheel brake to effect a brake application.
According to this invention, the master cylinder has a housing with a bore that is connected through axial port in the bottom of the bore and a radial port with a reservoir and to the brake system through first and second outlet ports. First and second pistons are positioned in the bore by a first resilient means located between the first and second pistons to define limits for a first chamber and by a second resilient means located between the second piston and the bottom of the bore to define limits for a second chamber. The first and second resilient means each include a return spring and a poppet spring with the first return spring caged between a retainer and a piston while the poppet spring is caged between the retainer and a poppet included in a center port compensation valve. The poppet is defined by a cylindrical member with an axial bore that extends from a first end to a second end. The axial bore has an annular shoulder adjacent a groove that retains a lip seal near the second end of the cylindrical body while an annular axial projection that extends from the first end of the cylindrical body retains a resilient sealing ring. A resilient ring is located on the annular axial projection while a radial projection that extends from the first end of the cylindrical body forms a surface for holding one end of the poppet spring engages a tubular body of the retainer concentrically locates the axial bore of the cylindrical body with respect to a seat of passage through which fluid is communicated from a reservoir. A shaft with a head on a first end and threads on a second end extends through the axial bore of the cylindrical member with the lip seal engaging the shaft. When the threads are mated with corresponding threads in the piston, the head is urged toward the annular shoulder and the return spring is caged between the retainer and piston to define limits for a chamber while at the same time the poppet spring is caged between the radial projection on the cylindrical body and the retainer. The piston on receiving an input force responds by moving to compresses the return spring such that the piston and shaft move together with respect to the retainer to permit the poppet spring to move resilient ring on the cylindrical body into contact with the seat surrounding the passage connected to the reservoir to interrupt fluid communication from the reservoir. With subsequently movement of the piston fluid in the chamber is pressurized and communicated to wheel brakes to effect a brake application. Since the shaft moves with the piston during the development of the pressurized fluid once the poppet spring has seated the resilient ring on the seat surrounding the passage, the head moves away from the shoulder and within the axial bore of the cylindrical member and should a maximum pressurization of fluid occur in the chamber through the movement of the piston may actually move into the passage without adversely effecting movement of the piston.
An advantage of this brake system is provided by permitting a head on a stem that cages a return spring to slide within a cylindrical member of a poppet of a compensation valve that controls communication between a reservoir and the bore of a master cylinder during the development of pressurizing fluid.
A further advantage of this invention is provided by caging a return spring between a retainer-poppet assembly and a piston wherein a head on a shaft is urged against a stop in a cylindrical member of the retainer-poppet when threads on the shaft are mated with corresponding threads in the piston and allowing the head to move within an axial bore of the cylindrical member when a face on the cylindrical member is seated by a poppet spring during pressurization of fluid to effect a brake application.
A still further advantage of the present invention resided in the reduction in space within a master cylinder where air may be trapped to assure that a system is completely filled fluid.